Embodiments of the invention relate generally to radiographic detectors for diagnostic imaging and, more particularly, to a Computed Tomography (CT) detector module configured to provide temperature drift correction capability.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector and rejecting scatter from the patient, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
In operation, each scintillator of a scintillator array converts x-rays to light energy and discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal, with the strength of the electrical signal being proportional to the total energy absorbed. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
In order to operate in an effective manner and generate high quality and artifact free CT images, a CT detector must meet tight performance requirements. First, the detector should provide a response that is linearly related to x-ray intensity. Some of the requirements on the detector that result from this are stability of the detector over time and temperature, non-sensitivity to focal spot motion, and a bound on the light output variation over life. Furthermore, for a third generation CT scanner, the relative behavior of adjacent channels of the detector should be nearly identical in order to avoid serious ring artifacts (usually defined as channel-to-channel non-linearity variation) in reconstructed images. This variation might be affected by the scintillator behavior from one pixel to its neighbor, by the collimator plate variations, and/or by the diode pixel response. Generally, if these requirements are not met, ring artifacts, bands and/or smudges/spots might appear in images.
One of the contributors of this channel-to-channel variation (or module to module variation) is the gain variation caused between detector pixels (composed of the collimator-scintillator-photodiode arrangement) due to the variation of temperature. In Volume CT, the variation of the temperature at the pixels will be high and tight thermal control from calibration conditions to scanning conditions is typically required. The thermal gain temperature coefficient drift in the detector module may have multiple root causes, including the diode, collimator, scintillator, and DAS electronics. To overcome this problem, very tight thermal control can be introduced on the detector or, alternatively, the thermal drift can be compensated for by introducing a thermal calibration or correction.
Therefore, it would be desirable to design a CT detector that minimizes gain variation between detector pixels resulting from temperature variation. It would also be desirable for such a CT detector to minimize such gain variation by providing a thermal calibration or correction to compensate for such temperature variation.